Electronics, microelectronics and microelectromechanics require as starting materials semiconductor wafers which have to meet extreme demands regarding global and local flatness, single-side-referenced flatness (nanotopology), roughness and cleanness. Semiconductor wafers are wafers composed of semiconductor materials such as elemental semiconductors (silicon, germanium), compound semiconductors (for example composed of an element of the third main group of the periodic table, such as aluminum, gallium or indium plus an element of the fifth main group of the periodic table such as nitrogen, phosphorus or arsenic) or the compounds thereof (for example Si1-xGex, 0<x<1).
In accordance with the prior art, semiconductor wafers are produced by means of a multiplicity of successive process steps which can generally be classified into the following groups:    (a) producing a usually monocrystalline semiconductor rod;    (b) slicing the rod into individual wafers;    (c) mechanical processing;    (d) chemical processing;    (e) chemomechanical processing;    (f) if appropriate additionally producing layer structures.
Advantageous methods in the context of steps (c) and (e) in this case include so-called “free floating processes” (FFP), in which both sides of a semiconductor wafer are simultaneously processed in material-removing fashion in one work step, to be precise in such a way that the processing forces acting on the semiconductor wafer on the front and rear sides during the material removal compensate for one another, such that the semiconductor wafer is processed in “free floating” fashion substantially without constraining forces of a guide apparatus being exerted. In this case, “substantially” means that, as a result of the kinematic characteristics of the process, the forces acting on the front and rear sides during processing can at least in principle precisely balance one another and that low resulting residual forces that may occur, will occur only on account of statistical fluctuations or external disturbance variables. By means of FFP, defects of form as a result of prior processes can be removed particularly effectively and with little material removal, and the FFP impress hardly any processing-characteristic new defects of form attributed to them on the semiconductor wafers.
In the prior art, preference is given to sequences for producing semiconductor wafers in which at least one of the process steps involved is an FFP. In the prior art, particular preference is given to sequences in which at least one FFP comprises a method in which both sides of at least two semiconductor wafers are simultaneously processed in a material-removing fashion between two ring-shaped working disks, wherein the semiconductor wafers are inserted loosely into in each case at least one receiving opening overall of at least one thin guide cage (carrier) toothed on the outside, which are guided by means of a rolling apparatus and the outer toothing under pressure on cycloidal paths relative to the working disks, such that they rotate completely around the midpoint of the double-side processing apparatus (planetary movement). Such kinematics are used for the lapping, grinding or polishing of semiconductor wafers.
US2009/0298396A1 and US2009/0298397A1 describe double-side grinding methods with planetary kinematics, which are intended to lead to a very flat surface without edge roll-off even in the case of semiconductor wafers having a diameter of 450 mm. In this case, a plurality of semiconductor wafers having the same diameter are arranged in a carrier on precisely one pitch circle around the midpoint of the carrier in such a way that the ratio of the area of the pitch circle to the area of a semiconductor wafer is between 1.33 and 2.0. US2009/0298396A1 furthermore makes certain requirements of the size and arrangement of the abrasive pellets used in the method. By contrast, US2009/0298397A1 describes a grinding method with the same arrangement of the semiconductor wafers in the carriers, but an alkaline solution is used in addition to the bonded abrasive, and the rotational speed of the semiconductor wafers is between 5 and 80 revolutions per minute. It has been found, however, that complying with these requirements does not suffice to obtain semiconductor wafers having the required plane-parallelism of the flat surfaces. Particularly in the case of an arrangement with only one large semiconductor wafer per carrier, sufficient flatness was often not obtained. With most of the commercially available double-side processing apparatuses, an arrangement like that cannot be avoided for example in the case of semiconductor wafers having a diameter of 450 mm, since the corresponding carriers are not large enough for receiving a plurality of semiconductor wafers of that size.
Other arrangements of semiconductor wafers in the carriers and of carriers in the double-side processing apparatus are also known. By way of example DE10159848A1 specifies that, in a double-side polishing apparatus mentioned by way of example, having a diameter of 1970 mm for the outer drive ring and 530 mm for the inner drive ring, it is possible to insert up to 5 carriers having a pitch circle diameter of the outer toothing of 720 mm. In each carrier there is space for three semiconductor wafers having a diameter of 300 mm. In general, however, the arrangements disclosed are not considered to be related to the plane-parallelism that can be obtained by the processing.
FIG. 8 shows the essential elements of an apparatus according to the prior art that is suitable for carrying out the method according to the invention. The apparatus is suitable for simultaneous double-side lapping, grinding or polishing with planetary kinematics. The illustration shows the basic schematic diagram of a two-disk machine for processing disk-shaped workpieces such as semiconductor wafers, such as is disclosed for example in DE10007390A1, in perspective view. FIGS. 1 to 4 show a plan view of the arrangement of the carriers 13 and of the openings 1 within the carriers 13. The reference symbols used in the following description of the apparatus and the way in which the apparatus works relate to these five figures.
An apparatus of this type consists of an upper working disk 31 and a lower working disk 32 and a rolling apparatus formed from an inner drive ring 33 and an outer drive ring 35, carriers 13 being inserted into said rolling apparatus. The working disks of an apparatus of this type are ring-shaped. The carriers have openings 1 which receive the semiconductor wafers 36. (Carriers each having three openings are illustrated. By contrast, the invention relates to carriers having only one opening.)
During processing, the working disks 31 and 32 and the drive rings 33 and 35 rotate at rotational speeds no, nu, ni and na concentrically around the midpoint 3 of the entire apparatus (four-way drive). As a result, the carriers on the one hand rotate on a pitch circle 8 around the midpoint 3 and on the other hand simultaneously perform an inherent rotation about their respective midpoints 4. For an arbitrary point of a semiconductor wafer, a characteristic trajectory (kinematics), results with respect to the lower working disk 32 and the upper working disk 31, said trajectory being referred to as a trochoid. A trochoid is understood as the generality of all regular, shortened or lengthened epi- or hypocycloids.
Depending on the type of processing method (lapping, polishing, grinding), the upper working disk 31 and lower working disk 32 can bear working layers 39, 40. They are polishing pads in the case of polishing, and working layers containing bonded abrasive in the case of grinding. The interspace formed between the working layers 39 and 40 is referred to as a working gap 30, in which the semiconductor wafers 36 move during processing.
At least one working disk, for example the upper working disk 31, contains holes 34 through which operating agents can be fed to the working gap 30, for example a cooling lubricant, a polishing agent or a lapping agent. In addition, measuring devices 37 for measuring the width of the working gap 30 can be present.
During lapping, a slurry of loose hard substances having an abrasive effect (lapping agent, lapping slurry) is fed to the working gap 30 and material removal from the semiconductor wafer 36 is effected in this way. The working surfaces of the working disks 31, 32 contain no abrasive in this case.
During grinding, by contrast, the working disks 31, 32 respectively comprise a working layer 39, 40 facing the working gap 30 and containing fixedly bonded abrasive. A cooling lubricant containing no substances having an abrasive action is fed to the working gap 30. The working layer can consist of an elastic abrasive pad containing fixedly bonded abrasive. This is referred to as a PPG method (“planetary pad grinding”). The abrasive pad is connected to the working disk magnetically, by vacuum, by hook-and-loop fastening or by adhesive bonding for the duration of use and can be removed by a peeling movement after use and thus be changed rapidly. Alternatively, the working layer 39 can also consist of a multiplicity of rigid abrasive bodies (so-called pellets). The abrasive bodies are embodied as cylinders, hollow cylinders or right prisms and by their end faces are adhesively bonded, screwed or incorporated into the surface of the working disk. Changing worn abrasive bodies is more complicated than changing the abrasive pads.
During polishing (double-side polishing, DSP), the working layers 39, 40 are polishing pads without any abrasive substances. A polishing agent (polishing slurry) containing abrasive substances, preferably a colloidally disperse alkaline silica sol, is fed to the working gap 30 formed between the polishing pads. The polishing pad is elastic and, similarly to the abrasive pad in the PPG method, can be removed from the working disk by means of a peeling movement and is therefore easy to change.